1. Field of the Invention
The present invention relates to a servo control apparatus, and in particular to a servo control apparatus equipped with a function of measuring mechanical lubrication characteristics on the basis of experimental modal analysis.
2. Description of the Related Art
Friction characteristics and vibration characteristics of a guiding mechanism in a machine tool will affect the operation performance of the feed axis and generation of chattering vibration at a machining stage. In such problems, the friction damping performance of the machine, in other words, the lubrication characteristics, have been commonly understood to be essentially important.
For example, it has been both experimentally and analytically clarified that improvement in stability against chattering vibration cannot be achieved by enhancing the stiffness of the machinery, but need to optimize both the stiffness of individual parts of the machinery and the damping characteristics (for example, Transactions of the Japan Society of Mechanical Engineers series C, Vol. 78, No. 787 (2012), pp. 1013-1025, which will be referred to hereinbelow as “non-patent document 1”).
It has also been reported that in an excitation test of a machine there exists excitation force-dependency and that the resonance frequency and damping ratio greatly vary depending on the magnitude of excitation force (for example, Yasunori Sakai et al., Journal of the Japan Society for Precision Engineering, Vol. 80, No. 8 (2014), p. 783-791, which will be referred to hereinbelow as “non-patent document 2”). The document suggests that the main reason for this is attributed to non-linear spring characteristics of the friction in a micro displacement area of some tens micrometers. There has been a recent report on a study in which the relationship between mechanical damping and non-linear friction was quantitatively evaluated by experiment (for example, Yasunori Sakai et al., JIMTOF 2014 poster publication “Influence of Excitation Force on Damping of Rolling Guideway in Feed and Pitch Direction”, which will be referred to hereinbelow as “non-patent document 3”). This publication demonstrates that the modal damping ratio (the half-value width of the resonance curve) becomes greater as the excitation force becomes greater, as a result of performing an excitation test on a machine to examine resonance characteristics of the compliance function (excitation force and displacement frequency response) by experimental modal analysis.